Transmission

Epidemiology
Host Range, Geographic Distribution and Epidemiology

Channel catfish (Ictalurus punctatus) is the most susceptible host but E. ictaluri is also known to be pathogenic for white catfish (I. melas), walking catfish (Clarias batrachus) and brown bullhead (I. nebulous), as well as ornamental fish such as the danio, rosy barb and green knife fish.

Some strains of channel catfish appear to be more resistance to ESC disease than others. The European catfish (Silurus glanis) and many other commonly cultured warmwater fish are considered resistant. The blue catfish (I. furcatus) is considered to be more resistant to the disease than channel catfish and, when hybridised with channel catfish, results in progeny with an intermediate level of resistance.

As a Gram negative pathogen, E. ictaluri appears to have a comparatively narrow host range but, under experimental laboratory conditions, it has been shown to be pathogenic for a wide variety of fish including trout, salmon, tilapia and other species of catfish and ornamental fish. However, natural outbreaks of ESC disease in these species have not yet been reported.

ESC is wide spread in the U.S.A and considered endemic to all waters with resident populations of catfish. However, E. ictaluri has also been reported in Thailand and Australia. Epizootic occurrences are most common in farm-raised stocks but have also been described in wild stocks. Given the relatively slow growth and fastidious nature of the organism, E. ictaluri may often remain undetected by diagnostic laboratories unfamiliar with its differential diagnosis and the true host range and distribution may be wider than currently thought.

ESC can occur as an acute or chronic disease depending on water temperatures as well as the species, size and prior disease history of the host. The disease tends to be most acute in fry and fingerling fish during the autumn of the first years of production when water temperatures are between 20 and 28 °C (68 to 82 °F), but more chronic infections and mortality can occur almost year around.

Survivors of moderate to severe outbreaks of ESC disease appear to develop a high level of acquired life-long immunity to the disease and subsequent or recurrent outbreaks of the disease in these fish are rare. However, E. ictaluri is also known to establish a clinically apparent carrier infection in surviving fish. These carrier fish are able to reservoir and transmit the pathogen through their life, most likely through infected faeces. E. ictaluri has also been shown to survive in water at 25 °C for up to 15 days and to remain infective in bottom mud for over 90 days at 25 °C.

Transmission is typically due to direct fish-to-fish contact, particularly when susceptible fish, such as fingerlings, are “under-stocked” in ponds together with larger fish carrying the disease.

Likewise, newly constructed earthen ponds tend to experience fewer outbreaks of ESC but the disease becomes more prevalent in later years, even though the ponds are drained, air dried and disinfected between crops.


Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.